twinlark: <Shams> <What was Bocharov's idea with the pawn sac <26...Qa3>?>
My guess is to bring the DSB in behind the queen onto a5 taking control of the dark squares behind White's lines. Problem is White has all the access squares covered and easily beats Black off, eg: assuming that he saw White's response <27. Bb4>, then after <27...Qa2 28. Qxa2 Rxa2 29.Nec1> and the rook has to retreat as all the squares on the seventh are covered. White then takes the pawn gift.
On the other hand, Bocharov is a 2600+ GM, so he may have simply over-estimated the value of the subsequent infiltration by his heavy pieces of White's position. For example, after <31...Ra2 32. Qb1> White is looking cramped and Black seems to have chances, but Black is quite lost if White plays correctly, which of course he did when he forces the win of the exchange on a2 a few moves later.
According to my engine, <26...Qa3> is the losing mistake and Dubov played impeccably to bring the win home.